This invention relates to electric measuring devices, and more particularly to a device for determining the power usage of an alternating current load.
Measurement of electrical circuit parameters, such as voltage, current, resistance, and power, is often required for monitoring a circuit for normal operation or for localizing a fault in the circuit. Many different types of instruments, varying over a wide range of price and complexity, are available to measure one or more of these parameters directly or to derive them from other measurements.
The simplest measurement devices are meters, which may be configured to measure voltage, current, resistance, or some combination of these parameters. An example of an ohmmeter is based on a d""Arsonval meter circuit, which consists of a meter in series with a battery and variable resistor. The meter has a pointer that moves in a manner proportional to current. The ohmmeter is calibrated by adjusting the variable resistor to show a zero resistance reading. Then when an unknown resistance is connected, the meter reflects the unknown resistance.
Digital multimeters combine a voltmeter, ammeter, and ohmmeter in a single device. The internal circuits of the multimeter convert the selected input variable to a dc voltage that is proportional to the measured value for the input variable. This dc voltage is converted to digital form and a numerical value is displayed.
Power has conventionally been measured with a wattmeter. A typical wattmeter has a pair of fixed current coils and a moveable coil. The current coils are in series with the load and have the same line current. The moveable coil is across the load and carries a needle that moves over a scale. Field strength of the current coils is proportional to the line current and in phase with it. The moveable coil produces a flux that is dependent on voltage. Because the force acting on the moveable coil is proportional to the time average of the product of the line current and the voltage, the needle deflection is proportional to power.
Other circuit parameters, such as impedance and reactance, may be derived from measured values. Various techniques for calculating these parameters are known in the art of circuit analysis.
One aspect of the invention is a device for measuring power parameters of an alternating current load, such as power and the power factor. An ammeter is connected in series with the load, and a voltmeter is connected across the load. A known impedance is also connected across the load in parallel with the voltmeter. The known impedance is switchable in and out of the circuit formed by the ammeter and voltmeter. The device also has means for connecting the circuit to an alternating voltage supply. A processor receives a voltage measurement from the voltmeter, a first current measurement from the ammeter when the known impedance is out of circuit, and a second current measurement from the ammeter when the known impedance is in circuit. The processor is programmed to use these measurements and the known impedance value to calculate the power usage of the load, the power factor, or both. A user interface has a display that displays the calculated value(s).
An advantage of the invention is that it provides a simple inexpensive alternative to a conventional wattmeter. It can be designed as a small compact device, convenient for measuring power consumed by any ac load. It is useful for measuring xe2x80x9cpower pollutionxe2x80x9d, such as that resulting from appliances and other consumer electronics that continue to consume power even while turned off.
The power measurement provided by the device is a xe2x80x9ctruexe2x80x9d ac power measurement. It is not simply the product of an ac voltage and an ac current, such as might be provided with a multimeter.